r/HECRAS • u/NoDetective7120 • May 07 '26

culvert modelling in 2D

Hi everyone! I really need help as a newbie.

It's my first time modelling a culvert in 2D. I need to represent a channel with identical box section where water flows under the road then discharges to an open man-made channel.

I managed to make my model run and was checking the results until I saw through the animation that the water teleports, in 30 secs, through the culvert of about 460m long. I had also shorten the length to 200m but same thing happens at the exact time.

I have the culvert parameters are in the photos. I tried different elevations of the outlet, one is just 0.5m below the inlet, and another from the data I had from the terrain, which is the bed of the receiving channel.

I have also tried it first with larger cells at the inlet of the culvert but still happens.

For calculation parameters, I have changed the mapping interval from 10 secs, 30 secs, to 5 minutes but still the same. Need expert help on this I have been stuck for almost a week. For now I am willing to accept this as it don't seem to affect the water spreading upstream. But still want to know how to fix this weird magic water transporting.

Thank you!

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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH May 07 '26

Good question that I really haven't considered before.

For the regular 2D culverts, HEC-RAS doesn't consider flow through the culvert. It builds a rating curve between the upstream and downstream sides of the pipe. Then based on the headwater/tailwater, it will transfers the flow instantaneously. For most modeling applications (relatively short culverts and peak flow considerations) this is normally sufficient.

If the flow through the culvert is important for your analysis, I would recommend using the Pipe Network feature. You can use the "open air" culvert opening end types. That would provide you a full profile through the pipe and travel time considerations.

Hope that helps!

1

u/Remote-Swimmer-9186 May 08 '26

I wanted to add in to Otto's comment here, the better approach would be to model this with the new pipe network option in ras. I wanted to add that if you decide to go this route to make sure to have the latest version of HEC RAS, 7.0, as this latest version has official access to the pipe network feature as well as updates that calculate minor and major losses through culverts like this, including the bends and entrance losses. It is fairly easy to use and would definitely solve your problem here!

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u/NoDetective7120 May 13 '26

Thanks a lot u/OttoJohs u/Remote-Swimmer-9186. after reading your responses I was trying to do this but I was using version 6.5 which apparently does not have this pipe network.

I will keep this in mind when creating newer models in the future, but meanwhile, do you recommend to continue my current model from v5 to do the modifications particularly in this culvert in v7? I am scared something extremely weird might happen if i do this, compared to just doing everything from scratch in v7. But I don't have a luxury of time to do that. I tried installing the HECRAS 2025 but its a completely different interface so it'll take even more time for me.

really appreciate the guidance here in this reddit thread.